The present invention relates generally to an exposure apparatus, and more particularly to a measurement of a polarization state of exposure light for the exposure apparatus that exposes a plate, such as a single crystal substrate for a semiconductor wafer, and a glass plate for a liquid crystal display.
A conventional projection exposure apparatus uses a projection optical system to project and transfer a circuit pattern of a reticle or a mask onto a wafer, etc., in manufacturing a semiconductor device in the photolithography technology.
Along with recent demands for fine processing to the semiconductor device, a high-resolution exposure apparatus is developed and exposes a pattern having a size of half of an exposure wavelength or smaller. In general, the high resolution is achieved by use of a shorter wavelength of the exposure light and an increase of a numerical aperture (“NA”) of a projection optical system. The increase of the NA of the projection optical system, which is referred to as high NA imaging, means an increase of an angle between a perpendicular to the image plane, and a traveling direction of the incident light.
The high NA imaging has a problem of a polarization of the exposure light. For example, assume exposure of a so-called line and space (L&S) pattern that has repetitive lines and spaces. The L&S pattern is formed by the plane-wave two-beam interference. Where an incident plane is defined as a plane including two beams' incident vectors, an s-polarized light is a polarized light perpendicular to the incident plane, and a p-polarized light is a polarized light parallel to the incident plane. A polarized light simply perpendicular to a paper plane may be referred to as the s-polarized light, and a polarized light simply parallel to the paper plane may be referred to as the p-polarized light.
When an angle is 90° between the two beams' incident vectors, the two s-polarized lights interfere with each other, forming a light intensity distribution corresponding to the L&S pattern on the image plane. On the other hand, the two p-polarized lights neither interfere with and cancel with each other, nor form the light intensity distribution corresponding to the L&S pattern on the image plane. The light intensity distribution with a blend of the s-polarized light and p-polarized light has a lower contrast on the image plane than that with only the s-polarized light. As the p-polarized light's ratio increases, the contrast of the light intensity distribution lowers on the image plane, and the pattern is not formed at last.
Polarization control over the exposure light is thus necessary, and fundamental researchs etc. are conducted. See, for example, Proceedings of SPIE, Vol. 5377 (2004), p. 68. An illumination optical system, more specifically, a pupil in the illumination optical system controls a polarization state of the exposure light. The exposure light, which has been polarization-controlled by the pupil in the illumination optical system, is irradiated onto a reticle via an optical system subsequent to the pupil in the illumination optical system, and projected and imaged by the projection optical system onto an image plane. The polarization-controlled exposure light forms a light intensity distribution having a sufficient contrast on the image plane, and realizes a fine pattern resolution.
Nevertheless, even when the pupil in the illumination optical system controls the polarization of the exposure light, the polarization state controlled at the pupil in the illumination optical system is not always maintained up to the image plane due to the influences of the optical system subsequent to the pupil in the illumination optical system and/or the projection optical system. Thus, developments of the illumination optical system and/or the projection optical system that work more ideally are promoted, improving characteristics of the illumination optical system and/or the projection optical system.
For all that, a polarization characteristic is highly likely to vary on assembly into the exposure apparatus due to the stress birefringence. It is thus necessary to measure a polarization state of the exposure apparatus. For example, a proposed measuring apparatus measures the polarization state of the exposure apparatus by calculating a Stokes parameter. See, for example, Japanese Patent Application, Publication No. 2003-329516.
In order to measure or restore the polarization state from the Stokes parameter, the measuring apparatus disclosed in this reference splits the exposure light into plural rays while maintaining the polarization state, and detects the plural split exposure lights, thus requiring light splitting means for splitting the exposure light into plural rays, and plural detectors, such as a CCD, for detecting the plural split exposure lights. When the measuring apparatus is installed in the exposure apparatus, the structure becomes very complex and the exposure apparatus needs a large installation space. Of course, it is sometimes difficult or impossible for an exposure apparatus design convenience to secure the space for the measuring apparatus. It is thus difficult to actually install the above measuring apparatus in the exposure apparatus, and there is a demand for a newly developed measuring apparatus that measures a polarization state of the exposure apparatus.